1. Field of the Invention
The present invention relates to a method of fabricating an electroacoustic transducer for converting electric input signals into sound.
2. Description of the Prior Art
Electroacoustic transducers are employed in various miniature electronic devices, such as card-shaped portable pagers. Miniaturization of electroacoustic transducers to be incorporated into such miniature electronic devices has been requested and efforts have been made for the further miniaturization of the component parts of electroacoustic transducers.
FIGS. 16 and 17 show an example of a conventional electroacoustic transducer. The construction of and a method of fabricating this conventional electroacoustic transducer will be described. A cylindrical upper case 102 and a cylindrical lower case 104 are formed individually of a synthetic resin by molding. A yoke 106 is attached to the lower side of the lower case 104 and a base plate 108 is attached to the lower surface of the yoke 106. A core 110 has a lower end fixedly inserted in a hole formed coaxially through the yoke 106 and the base plate 108 so that the former is caulked by and integrated with the latter. A coil 112 which is wound previously around a bobbin, etc. is mounted on the core 110, and an annular magnet 114 is disposed so as to surround the coil 112. Leads 116 and 118 of the coin 112 extend outside from the back side of the base plate 108 and are soldered to electrodes 120 and 122 formed on the base plate 108, respectively. The yoke 106, the core 110 and the magnet 114 form an electromagnetic transducing portion 126 for driving a diaphragm 124.
The lower case 104 has a supporting stepped portion 128 near the upper open end thereof on the inner surface thereof, and a large diameter portion 130 formed at the upper open end of the lower case 104 to form the supporting stepped portion 128. The diaphragm 124 is seated on the supporting stepped portion 128. A circular magnetic piece 132 is attached to the central portion of the diaphragm 124 to increase the vibrating mass of the diaphragm 124.
An engaging portion 134 of the upper case 102 is fitted in the large diameter portion 130 of the lower case 104, and the upper case 102 and the lower case 104 are joined together in an integral unit by joining means, such as ultrasonic welding. Thus, a resonance space 136 that resonates with the vibration of the diaphragm 124 is formed in the upper case 102 to produce an appropriate sound pressure. A sound emitting cylinder 140 coaxially having a through hole 138 projects into the resonance space 136 from the inner surface of the top wall of the upper case 102.
This electroacoustic transducer has a basic construction for converting electric input signals into sound and comprises a comparatively large number of independent component parts. Accordingly, the electroacoustic transducer must be assembled very carefully which obstructs an automatic assembly thereof because the characteristics of the electroacoustic transducer including a sound output characteristic are greatly dependent on the accuracy of alignment of the component parts, particularly that of alignment of the diaphragm 124 and the magnet 114.
In assembly of component parts of such an electroacoustic transducer, when the component parts are mounted on the lower case 104, such mounting is carried out in a state where the lower case 104 is positioned as illustrated in FIG. 16, namely, the open end thereof is directed upward, and also such mounting is carried out in a state where the lower case 104 is reversed, namely, the open end thereof is directed downward. That is, the lower case 104 need be reversed in each step in such a way that firstly the yoke 106 and the base plate 108 are attached to the lower case 104, secondly the magnet 114 is attached to the lower case 104 while the lower case 104 is reversed. Thirdly the adhesive is applied to the attached portions of the yoke 106, the base plate 108 and the magnet 114 while the lower case 104 is again reversed, fourthly the diaphragm 124 is seated on the lower case 104 and the upper case 102 is fitted in the lower case 104 while the lower case 104 is further reversed, and finally the upper case 102 and the lower case 104 are joined together by the ultrasonic welding. Such a mechanical reversal of the lower case enhances accuracy of assembly and accomplishes uniform quality. However, this makes the makes process jigs and process steps complex.